Residual Stresses in Textured Zirconium Alloys

The elastic, plastic, and thermal properties of zirconium-base alloys are highly anisotropic. Consequently, thermomechanical treatments will leave fabricated components with high residual stresses. These stresses play an important role in determining subsequent mechanical properties and the dimensional stability of reactor-core components. When a component is deformed or subjected to a change in temperature, the total strain in any given grain will be accommodated elastically, plastically by shear on the active slip systems or by twinning, and by thermal expansion. The partitioning among the three modes depends not only on the particular orientation of the grain but also on the orientation of the other grains (texture) in the component. A self-consistent analysis, based on the concept of the hcp single-crystal yield surface, has been developed to predict the way that grains in a textured polycrystal respond to arbitrary external thermal or mechanical loading. The calculation follows the elastic-plastic evolution of strain inside each grain, taking into account the anisotropic elastic constants and thermal expansion coefficients. Workhardening due to shear on the active slip systems is included explicitly in the analysis.

Specific examples of the residual stresses produced by tension-compression deformation and thermal cycling of Zircaloy-2 rod will be described. It will be shown that the elastic-plastic transition in Zircaloy-2 requires much larger uniaxial strain and results in much higher apparent workhardening rates than in fcc metals. The predictions will be compared with experimental measurements of residual stresses made using the technique of neutron diffraction.